The manufacture of membrane sensors is carried out by depositing the membrane materials, made of silicon oxide, silicon nitride, polysilicon, or metals, for example, on a silicon wafer and subsequent local wet or dry chemical etching of the silicon from the backside up to the membrane. For reducing the heat conduction into the substrate, these sensors frequently have a very thin membrane.
Such a sensor is described in published German patent document DE 42 15 722, for example, in which the membrane is provided with an elastic enclosure for reducing the sensitivity toward the occurring notch tensions on the membrane mounting and for improving the pressure sensitivity. The elastic enclosure is attached opposite the side of the membrane on which the measuring elements are situated and where the medium to be measured flows past.
In these types of sensors having direct media contact, such as the above-mentioned mass air flow sensor, air pressure sensor, or air quality sensor, the membrane, having a thickness in the micrometer range and lateral dimensions from a few 100 micrometers to several millimeters, is directly exposed to the flowing medium, e.g., air. The particles such as dust, contained in the media despite the use of filters, may thus cause damage to the membrane due to the direct bombardment of the membrane with the particles at relatively high flow speeds. Likewise, unfavorable pressure conditions may result in the fact that the membrane of an air pressure sensor experiences too great a deflection. It has been found that, due to the cases indicated, the membrane is susceptible to damage mainly at the break-sensitive membrane rim. Damage to the membrane may range from impairment of the measuring sensitivity to total failure due to the sensor's destruction.
The stability of the membranes against particle bombardment is determined via the statistical service life in an air flow having a defined dust quantity. An increase in stability could be achieved by increasing the membrane thickness and/or by lowering the pre-stressing of the membrane; an increase in the thickness, however, results in an increase in heat conductivity and thus in a deterioration of the characteristic curve. As a rule, pre-stressing cannot be sufficiently lowered since, to prevent bulging, the membrane must remain in the tensile stress range under all operating conditions.
The membrane thickness in air pressure sensors may also be varied only to a certain degree since thickening of the membrane involves the displacement of the measurable pressure range.
German patent document DE 102 10 335 describes a membrane sensor whose membrane is situated in a mounting on the sensor housing. In addition to this mounting, the sensor has an additional layer on the membrane rim with which the stability of the membrane against impacting particles is increased.